Hi Craig,
Here are general guidelines:
http://msdn.microsoft.com/en-us/library/windows/apps/hh464920
Use the templates provided with Visual Studio for the Metro feel! Guidelines on Metro style apps etc...
http://channel9.msdn.com/Events/BUILD/BUILD2011/BPS-1004 - I recommend watching the entire presentation. The first section (Metro style design) discusses layout.
Later on he around 19 it talks about placement of items.
You can also create a default HTML Grid app and open in Expression to see what CSS rules are applied to the elements, or a C# or VB Grid app to see positioning in the XAML.
Documentation is still being published on may topics and stay tuned for more as we go through this Developer Preview!
-Jeff
Jeff Sanders (MSFT)